KeyMint: tweak spec to allow for no v4 in V

Given that we are not expecting to release a v4 of the KeyMint HAL for
Android V, tweak some spec and test details so that existing v3
implementations do not need to change.

- Soften the requirement to use (1970-01-01, 9999-12-31) as cert dates
  when secure-importing an asymmetric to be a suggestion instead.
- Change the version gate for the test of importing an EC key with no
  specified `EC_CURVE` to be VSR-gated rather than gated on a putative
  future version of the HAL.

Test: VtsAidlKeyMintTargetTest
Bug: 292318194
Bug: 292534977
Change-Id: Ib8d6e79ea948ee77eeb2528d698205179f026fd3